Our objective is to develop an assay system which will sensitively and reliably differentiate between mutagenic and non-mutagenic halogenated hydrocarbons. Many of the carcinogenic alkyl halides fail to exhibit mutagenic activity when assayed in the Salmonella auxotroph reversion test. This may be due to an inability of the highly-lipophilic, alkyl halide metabolites to pass through the Salmonella cell envelope. We are therefore proposing to assay the ability of carcinogenic alkyl halides to induce mutagenic lesions in isolated DNA. The presence of these lesions will be determined by transformation and transfection assays using Bacillus subtilis as the host cell system. A series of auxotrophic tester strains carrying linked base substitution and frame shift mutations will be constructed to monitor the capacity of the test compounds to revert these mutations. The induction of forward mutations will be assayed by marker rescue with DNA from phage phi 105. The proposed system should prove useful not only as a pre-screen to detect potentially carcinogenic alkyl halides but also for assaying compounds which are too toxic to assay in whole cell systems.